The first thing you want to do is to download your phones stock rom, stock rom is the rom which comes with your phone for the very first time, ''(Factory rom)''
If your able to go into download mode you should connect your phone (your in download mode now) to the computer and open the odin program, flash the original rom into the phone!
Your phone is now as it was when you bought it

This worked for me when I flashed the wrong kernel on my S3 lte, I flashed Siyah kernel which is for the i9300 version
Then I tried this method! And it worked!